The present invention relates generally to the field of block copolymers. More particularly, it concerns monovinylarene-conjugated diene copolymers useful in shrink film applications, especially in blends with polystyrene.
Articles formed from monovinylarene-conjugated diene copolymers, such as styrene-butadiene copolymers, for example K-Resin® (Chevron Phillips Chemical Company LP, The Woodlands, Tex.), generally have improved physical properties compared to articles formed from general purpose polystyrenes. However, in the case of articles for which heat shrink performance is important, new monovinylarene-conjugated diene copolymers are necessary to provide the shrink performance desired by this growing market. As an example, typical monovinylarene-conjugated diene copolymers possess glass transition temperatures (Tg), which are the primary controller of shrink performance, that are typically in the range of about 95° C. to about 108° C. This relatively high Tg is not favored by the marketplace as a relatively high temperature is requires to initiate shrinking.
Further, polystyrene is commonly blended with monovinylarene-conjugated diene copolymers for a wide number of reasons including increase film stiffness and decreased costs. Articles formed from blends of polystyrene and monovinylarene-conjugated diene copolymers can also be used in applications where heat shrink performance is important.
Therefore, it would be desirable to have monovinylarene-butadiene copolymers with lower Tg and improved heat shrink performance, either alone or in blends with polystyrene.